1. Technical Field
The present invention relates generally to bowling balls, and more particularly, a bowling ball having a novel mass distribution for establishing an enhanced preferential spin axis such that the bowling ball exhibits predictable dynamic characteristics.
2. Background Art
Bowling balls used in events sanctioned by the American Bowling Congress must conform to certain specifications. For example, the ball cannot weigh more than 16 pounds and must have a circumference of between 26.784 and 27.002 inches. The ball is normally provided with appropriately spaced finger holes for reception of the thumb, middle finger and ring finger. The geometric center of the drilled holes for gripping define the "Top" of the ball. After the ball is drilled, it must be balanced statically in three planes as follows: 1 onuce + or- from the left half of the ball to the right half through the vertical midplane which passes through the center of the grip, 1 ounce + or- from the finger to the thumb half of the ball through the vertical midplane which passes through the center of the grip perpendicular to the first plane, and 3 ounces + or- from the top to the bottom half of the ball through the horizontal midplane referenced to the grip center.
Skillful bowlers roll the ball so that it enters the pin placement at an angle with respect to the longitudinal axis of the bowling lane. It is known that larger angles of entry results in a larger area of impact to result in a strike or knocking all ten pins down with one roll of the ball. A skillful bowler can maximize this entry angle by throwing the ball so that it follows a curved path down the lane as it approaches the pins.
To achieve the desired curved path of the bowling ball, the bowler applies several components of motion. The first is the translational velocity of the ball towards the pins. The second is a combination of forward rotation (parallel to the longitudinal axis of the lane) and side rotation (perpendicular to the longitudinal axis of the lane). Skillful bowlers vary the ratio of forward rotation to side rotation in order to regulate the amount of hook or curve imparted to the ball. The friction between the ball and the lane surface combined with the rotation of the ball creates the curved path of the ball.
It is generally known in the field of bowling balls to incorporate weight blocks into the design thereof to compensate for the weight removed by drilling the grip holes. The weight blocks are traditionally placed in the "top" of the ball and provide a small degree of dynamic balance to the ball as it rolls down the lane. It is also known that by manipulating these weight blocks, a higher degree of hook can be obtained without violating the rules which govern the balance of the ball. This additional hooking action on the ball results from the dynamics associated with an object in rotation.
Turning to the laws of physics, a freely rotating object tends to rotate about its most stable axis of rotation. The most stable axis of rotation is the axis about which the object possesses the maximum moment of inertia, or the largest resistance to rotation This axis is referred to as the principal, primary, or dominant axis of rotation. The second most stable axis is the axis about which the object possesses the minimum moment of inertia, also a principal axis known as the secondary axis. If a rotational force or torque is initially applied to the object in such a manner as to induce rotation about an axis which is not a principal axis of rotation, the axis about which the freely rotating object will spin will gradually migrate from the original axis to the primary axis associated with the maximum moment of inertia of the object. Rotation of an object about the secondary axis is not ultimately stable therefore any disturbance of the rotation will also cause the migration to the primary axis
The same physical behavior described above applies to the dynamic response of bowling balls. A bowler releases the ball in such a manner prescribed by the rotational force or torque applied at release to rotate about a particular axis on the ball. As the ball moves down the lane, the axis about which the ball is spinning tends to migrate toward the principal axis associated with the maximum moment of inertia of the ball.
While the ball is traveling down the lane, oil, which is applied to the lane to protect the surface, adheres to that portion of the ball which contacts with the lane, thereby reducing the friction of the interactive surfaces. During the migration of the spin axis, the ball rotates in such a fashion that the portion of the ball which does not contact the lane during the early parts of the ball trajectory, and thus remains "dry" comes into contact with the lane at points further down the lane. When the "dry" ball surface comes into contact with the lane surface, the frictional forces are maximized which results in a larger curved path. Without spin axis migration, this reduced friction will result in a smaller curved path that is less than that of a ball with induced spin axis migration.
The problem with current bowling ball design is that the primary axis of rotation is not strong enough at the time of manufacture to remain the primary axis after the ball is drilled, due to the alteration of moment of inertia caused by the drilling of the grip holes. In other words, the difference between the maximum and minimum moments of inertia is not large enough to remain in the same orientation when the grip holes are introduced. Current bowling ball designs are marginally stable about their minimum moment of inertia, as such they possess a line of minimum moment of inertia, and a plane of potential maximum moments of inertia near the horizontal midplane of the ball based on the removal of material during drilling. Because the axis of release is subject to the variability of a particular bowler, and because the primary axis of the ball does not correspond to the release axis of the bowler, the corresponding dynamic response is subject to a higher degree of variability as the spin axis migrates from the different intermediate axis to the primary spin axis.
Randolph U.S. Pat. No. 3,865,369 and Gentiluomo U.S. Pat. No. 4,882,671 both illustrate bowling balls having a top weighted hemisphere with a stable axis only about the minimum moment of inertia. As described above, these designs result in largely unpredictable responses.
Prior art fails to disclose a bowling ball having a differential moment of inertia large enough to maintain the maximum moment of inertia as the maximum moment of inertia after the removal of material for the grip holes. The present invention is directed toward overcoming one or more of the problems set forth above.